Method of producing rare metals by electrolysis



Patented June 7,- 1932 UNITED STATES PATENT OFFICE FRANK H. DRIGGS, OF BLOOMFIELD, AND WILLIAM C. LILLIENDAHL, OF MONTCLAE, NEW JERSEY, ASSIGNORS TO WESTINGHOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA METHOD OF. PRODUCING RARE METALS BY EECTBOLYSIS Ho Drawing. i

the rare refractory metals, such as thorium,

uranium, zirconium, tantalum, chromium, manganese, and the l1ke metals.

In the preparation of the rare refractory metals by electrolytic decomposition of fused salts it is inexpedient because of the relatively high melting points of the metals to effect deposition thereof in the form of an adherent plate or coating on a cathode surface. It is customary therefore to effect deposition of these metals at the cathode in the form of metal powders, which are recovered from the bath in any suitable manner, such as by dissolving away the aqueous and acid soluble materials. The metal powder is then freed of moisture, compacted into suitable shape and size, and consolidated into coherent metal by proper heat-treating methods.

Owing to the high chemical activity of the rare refractory metal powders difliculty has heretofore been experienced in protecting the metal powder from reacting with atmospheric gases and metalloidal impurities during the electrolysis procedure to form compounds therewith which may not be subsequently removed by aqueous and acid solution methods or by heat-treatment means. Such impurities when present in amounts greater than .5 to 1.0 per cent deleteriously affeet the subsequent consolidation of the compacted metal powders and usually alters the physical characteristics of the sintered rare metal article to such an extent that an extremely' ductile metal may be rendered totally unworkable.

One of the objects of the present invention is to provide a method of producing rare refractory metals substantially free of deleterious impurities.

Another object of this invention is to provide an electrolytic process of producing rare refractory metal powders substantially free of deleterious impurities.

Another object of this invention is to provide an efficient commercial method for elecv 1 Application filed March 30, 1929. Serial No. 351,451.

trolytically producing the rare refractory metal powders in a substantially pure state.

Another object of this invention is to provide a method of preparing rare refractory metal powders by electrolysis of fused salts using a free floating cathode.

Another object of this invention is to provide a method of producin a substantially reducing atmosphere about a cathode of the floating type during the electrolytic production of rare refractory metal powders.

Other objects and advantages will become apparent as the invention is more fully disclosed.

In accordance with the objects of our invention we have determined that the greater portion of the accompanying deleterious impurities which it is desired to eliminate are producedor formed during the electrolytic process of producing the metal powders. The subsequent lixiviation methods of freeing the metal powder from the associated electrolyte and the vacuum heat treatin process of consolidating the recovered metal powders into coherent metal bodies do not introduce when properly performed a sufficient amount of such impurities as to deleteriously affect the working qualities of the metal body.

In copending applications Serial No. 275,264 filed May 4, 1928; Serial No. 277,096 filed May 11, 1928; Serial No. 309,682 filed October 1, 1928; and Serial No. 316,624 filed November 1, 1928 b Frank H. Driggs who is the same Frank Driggs co-inventor of the present invention are disclosed methods of producing rare refractory metal powders by the electrolytic decomposition of their fused salts. These copending applications are all assigned to the sameassignee as the present invention.

These copending applications are directed broadly to methods of producing rare refractory metal powders by the electrolysis of fused baths containing at least in part a dissociable ionized compound of the rare refractory metal, and the bath composition is comprised of flux materials which are reactive with res ect to the oxygen compounds of the rare re ractory metals. The electrolytic process contemplates using an electricallly conducting container which may be externa ly heated to effect fusion of the contained electrolys1s method.

The methods disclosed in these copending applications have been highly successful m the preparation of rare rpfractory metal powders but they offer on commercial appllcation certain disadvantages which it is one of the objects of the present invention to eliminate.

The usual procedure in the recovery of the rare metal powders produced by these methods is to allow the electrolyte to cool 1n situ in the electrically conducting container, in

order to prevent access of atmospheric gases to the highly reactive metal powder while at elevated temperatures.

The cooled fusion mixture is then lixiviated with water and the recovered metal powder, washed several times with dilute acidsto remove acid soluble residues. Thewashed metal powder is recovered in any suitable manner freed of moisture, dried and is then subjected to heat-treatment to consolidate into sintered bodies.

It is found however that when the fused electrolyte is allowed to cool in situ in the container, it is often difficult to remove from the container and occasionally it is necessary to subject the cooled material to a prolonged soaking in water to effect complete disintegration thereof, in which soaking? operation the entire container may be submerged in the water. With graphite or carbon containers this soaking is deleterious to the useful life of the container as the porous material absorbs large quantities of the aqueous solution of the electrolyte and the crucible must subsequently be carefully dried out before it may be used again. Furthermore there appears to be a certain amount of disintegration of the container and portions thereof are separated out with the metal powder.

We have found that a great deal of the trouble in removing the cooled fusion from the container is caused by the fact that the deposited rare metal is present as a porous sponge material, the individual particles of which are united to each other and to the sides of the container with an electrically conducting bond, which requires a certain amount of applied force to break.

When a floating cathode is employed it is found that the metal powder is similarly deposited and retained adherent to the cathode as a metal sponge, which progressively grows in diameter as the electrolysis proceeds.

This sponge is suificiently adherent to the electrode so that upon completion of the electrolysis the same may be withdrawn from the bath, and being coated with an adherent film of the fused salts the metal powder is substantially protected from reaction with atmospheric gases. On cooling the sponge-like material may be placed in water and rapidly disintegrated and washed free of water and acid soluble impurities in a relatively short interval of time.

It is also found that such material is not contaminated by inert materials such as flake graphite carbon or fragments of crucibles which heretofore have been encountered by the prior methods and from which it is extremely diflicult to separate the metal powder.

We have found however that when a floating cathode is employed the operating temperature of the bath must be materially lower than what heretofore has been successfully used.

The exact reasons for this requirement are not at this time apparent but we have found that electrolytic baths of the type described in the copending applications above identified do not successfully or satisfactorily produce a metal powder of the required high degree of purity desired for the purpose of this invention, and the eificiency of the process is much lower than by the method as will hereinafter be described.

We believe this to be due to the fact that in the preparation of rare refractory metals is ap ears essential that in order to maintain a hig degree of purity and high cathode or production efiiciency, the deposited sponge of metal powders adherent to the cathode must be protected from interaction with the surrounding flux or fusion mixture or from interaction with the gases liberated at the anode.

With the use of a floating anode this latter eflect is negligible, however the rate of reaction with the surrounding fusion mixture or the solution pressure of the deposited metal appears to increase rapidly with the increase in temperature and the certain loss in cathode efiiciency will be experienced.

We have found that the reaction between the flux and the depositing sponge metal, on the cathode and interaction of the deposited sponge metal with atmospheric gases and associated metalloids to form deleterious compounds may be substantially prevented by providing means of liberating at the cathode simultaneously with .the depositing rare metal a material or substance which is more reactive chemically than the depositing rare refractory metal and which subsequently may be eliminated in the water or acid washes, or in the heat-treatment process of forming a coherent metal body. Such materials contemplated within the scope of the present invention are those metals higher in the electromotive series than the depositing rare refractory metal which would other- V magnesium, aluminum and the like.

Of these the most efiicient for the purpose of our invention are the alkali metals,li thium,

sodium, potassium, rubidium, caesium, which are the most highly reactive strongly basic elements, and which do not form alloyed compounds with the rare refractory metal as does zinc, magnesium and aluminum or some of the strongly basic rare earth metals. Such alloys as may be formed through the use of magnesium, zinc and aluminum may subsequently be thermally decomposed by heat-treatment in vacuo. For the purpose of the present invention however it is preferable to employ a highly reactive metal which does not alloy with the rare metal.

We have also determined that it is preferable that the alkali metal be present at least in part in the vapor state as a finely dispersed metal fog or vapor in the fusion adjacent and surrounding the rare metal sponge on the cathode. This requires that the temperature of the bath must be regulated and controlled so that the alkali metal is being liberated at the cathode at a temperature substantially above the melting point but materially below the boiling point of the alkali metal.

. The melting and boiling points ofthe al- In the practice of our invention and in accordance with the objects of our invention we have preferred to employ potassium, so: dium or lithium-and specifically prefer to employ sodium, as the halide compounds of this element are the most readily obtained on the market and arethe cheapest.

. In order, to retain the liberated sodium in the bath the temperature of the bath must be maintained below 87 7 .5 C. the boiling point and above 97.6 the melting point of the sodium and have found that for the purpose of this invention the proper operating temperatures of the bath are between 500 C. to 800 C. depending in part upon the particular rare refractory metal being prepared. We have found that it is not essential that the flux be comprised wholly or entirely of alkali metal compounds but that it is preferable to utilize a flux comprised in part of an alkali metal halide compound, such as sodium chloride, in part of an ionizable rare metal compound and in part of an alkaline earth metal halide compound such as calcium chloride, in such proportions as to give a bath having the desired melting point.

W e therefore utilize as a fusion mixture, admixed proportions of alkaline earth metal halides, and alkali metal halide compounds which will give a melting point within the required temperature limits. To this fusion mixture is added the proper halide or double halide compound of the rare refractory metalwhich it is desired to produce, such as the double alkali oralkaline earth metal chlorides, fluorides and the like compounds of the rare refractory metals.

The amount of ionizable rare metal com- I pound that is added to the bath is consistent with the desired specific temperature requirements of the bath. During the course i of the electrolysis further additions of the rare refractory metal compound or of the fusion mixture may be made from time to time, to'make the process essentially continuous in operation. It is also possible when the cathode has been covered with a sufficient amount of the rare refractory metal sponge to remove the same from the bath and replace with a fresh cathode.

The cathode employed may be of any noncorroding, non-contaminating material and will depend somewhat upon the particular rare metal being deposited. Uranium for instance readily alloys with nickel or nickel chromium alloys and when the cathode is comprised of these metals it is found that there is suflicient solution pressure of these metals in the bath so that appreciable amounts of the nickel or of the chromium is present in the deposited sponge metal. Thorium however will not appreciably alloy 86 therewith and a nickel or nickel chromium cathode may be used.

However, we have found that the most satisfactory cathode material is molybdenum, in sheet or in wire and rod form, and cathodes formed with such material are recommended for general application of the present invention.

The anode may be comprised of a graphite rod floating in the bath composition or a graphite crucible may be employed and used as the anode. It is preferable to employ a graphite crucible and utilize the crucible as the anode, as this provides a means of substantially eliminating the so-called anode efi'ects heretofore removed by the addition of metal oxide to the fusion mixture. Noncorroding 'metal anodes may be used but are not to be recommended due to the fact that the corrosive action of the halogen gases upon the anodeis extremely difficult to prevent and contamination of the depositing rare metal is usually obtained.

As a specific embodiment of the practice of our invention we will disclose the method employed when it is desired to produce uranium metal powder utilizing in the bath the metal sodium as the protecting reducing cathode metal fog therein.

- We prefer to employ calcium chlorlde as the alkaline earth metal halide component of the bath, and the specific admixture of alkali metal halide and alkaline earth metal hal de used depends in part upon the rare refractory metal which it is desired to produce and upon the particular halide or double hahde employed, and upon the specific fusion temperature of the three components of the bath.

lVhen calcium chloride and sodium chloride is employed as constituents of the bath the lowest melting eutectic mixture that may be obtained is at approximately 51.5% CaCh (in molar having a melting point of 505 C.

In the preparation of uranium a fusion ployed in the above identified copending application. Any other quatra-valent halide compound of uranium may be employed but we have found that this particular compound which may be so readily and easily prepared in an anhydrous oxide free condition is most serviceable from a commercial and economic standpoint. The particular amount by weight of this compound that is added depends in part upon the apparatus employed, diameter of crucible, desired amount of metal to be prepared, etc.

A standard type bath which we have used in the preparation of uranium is as follows:

Grams Calcium chloride (anhyd. C. P.) 250 Sodium chloride (anhyd. C. P.) 250 Potassium uranium fluoride (anhyd.) 30

When the entire fusion mixture is in quiet fusion the cathode comprised of a molybdenum wire, rod or sheet is submerged below the surface of the fusion and centrally disposed therein wtih respect to the surrounding graphite crucible walls which are made the anode of the bath, and a current passed through the bath at approximately 6 to 7 volts suflicient in amount to give a cathode current density of approximately 150 amperes per square decimeter. This particular current density is given by way of illustration and may be widely varied from 30 to 3000 amperes per square .decimeter depending upon operating conditions, without departing essentially from the nature of our mventlon. The principal effect of increased 'A' thin surface film of the fusion mixture is thus retained over the cathode which seals the same from contacting with the atmospheric gases.

If desired additional amounts of the rare metal salt (KUF or added increments of the flux may be made to the bath, a new cathode introduced therein and the electrolysis continued. If desired these additions may be made during the progress of the electrolysis and the same made substantially a continuous one.

On cooling, the spongy material on the cathode is subjected to the customary water and acid washes to remove the soluble material. Care should be taken't'hat' the sodium particles included in the sponge do not react too vigorously with the water during the washing process or that the liberated hydrogen does not explode.

After thorough washing with water and dilute acid the metal powder should be washed successively with alcohol and then with ether to remove the water and alcohol, following which it may be thoroughly dried in vacuo.

With highly reactive metals such as uranium, care should be taken that the dried metal powders be protected from rapid oxidation following degasification, otherwise such metal powdersare apt to react with air with explosive violence.

The dried powders may then be compacted into any desired shape and heat-treated in a high non-striking vacuo to effect sinterin or fusion to coherent bodies.

Whereas we have disclosed the specific process we employ in poducing uranium metal powder, it is obvious that instead of the uranium double fluoride compound, thorium double fluroide may be used, or we may use any of the rare refractory metal double halides such as the KCLCrCh compound in the preparation of chromium by this process, and that the beneficial effects obtained through the use of a protecting alkali or other highly reactive metal fog or deposit being liberated simultaneously with the depositing rare metal will be obtained irrespective of the rare metal being formed provided the temperature of the bath is maintained substantially below the vaporization point of the depositing reactive metal.

We have also found that the calcium chloride-sodium chloride fusion mixture may be utilized as a fusion mixture for all rare refractory metals, irrespective of the particular halide or double halide compound of the rare refractory metal being employed, and the temperature of the bath may be substantially regulated by restricting the amount of the said halide or double halide compound being added so as to keep the temperature of the bath below approximately 800 C. Such a fusion mixture is also more readily soluble in water and relatively easily and cheaply ob tained upon the market, effecting thereby a markedreduction. in the costs of operating and producing rare refractory metals by electrolytic methods. To this fusion mixture there'may be added anyother desirable compound of a highly reactive metal, if desired, such as magnesium chloride, aluminum chloride, and the like chlorides to effect substantially the same purpose as is produced by the use of an alkalichloride.

We have noticed also that under continuous operating conditions the fusion mixture tends to become viscous when an accumulation of fluorides in the fusion mixture is eflected. This viscous condition may be substantially overcome through the addition to the bath of proportions of a compound such as ammonium chloride, ammonium fluoride and the like low melting point halide salts. The word alkali as used throughout the specification and claims is used in its generic sense.

Having broadly outlined the scope of the present invention and specifically disclosed .one method of applying the same it is ap parent that there may be many variations in the specific materials employed and the meth-.

0d of operation disclosed without essentially departing from the nature of the present inventionv as covered in the following claims:

What is claimed is: i V 11. The method of producing uranium which comprises electrolytically depositing the metal from a fused bath of alkali halides containing'a quadrivalent uranium halide.

2. The method of producing uranium which comprises electrolyticallydepositing the metal from a fused bath of alkaline earth halides containing a quadrivalent uranium halide. I l

'3. The method of producing uranium which comprises electrolytically depositing th'e metafl from a fused bath of alkali and alkaline earth halides containing a quadrivalent uranium halide.

4. The method of producing uranium which comprises electrolytically decomposing a fused bath comprised of alkaline earth metal halides, alkali metal halides, a proportion of an ionizable compound ofa quadrivalent uranium halide, the operating temperature of said bath being such that a proportion of the alkaline metal component-liberated at the cathode as a result of said electrolysis is substantially retained in the fusion mixture adjacent the cathode as a metal fog. 5. The method of producing uranium which comprises electrolytically depositing the uranium from a-fused bath of calcium chloride and sodium chloride in approximate.

chloride in approximately equal proportions by weight and a proportion of anhydrous, oxide .free quatra-valent uranium halide compound.

8. The method of producing uranium which comprises electrolytically depositin the uranium from a fused bath comprise of an admixture of an alkaline earth metal halide, alkali metal halide, and a proportion of a uatra-valent uranium compound ionizable t erein, the proportions of said fusion mixture being such that the temperature of the bath is substantially below the vaporization temperature of the alkali metal component of the bath.

9. The method of producing rare refractory metals which comprises electrolytically depositing in the form of a powder said metal from a fused bath onto a cathode of the floating type. 3

10. The method of producing rare refractory metals whichincludes providing a fused bath containing a rare refractory metal compound, electrolytically depositing in granular form the rare metal from said bath upon a cathode of the floating type.

11. The method of producing rare refractory metals which includes fusing a bath containing a rare refractory metal compound in a crucible provided with a floating cathode and electrolytically depositing in granular tained in an electrically conductive crucible provided with a. cathode of the floating type, said crucible forming the anmie.

In testimony whereof we have hereunto subscribed our names this 28 day of March,

FRANK H; DREGGS.

m m a. LILLIENDAHL. 

